The present invention relates generally to monitoring devices, and more particularly to a novel bell mouth monitoring cassette for monitoring airborne particulate such as asbestos fibers and the like.
Recognition of the dangers of asbestos and the need for its measurement and control developed in the 1930's. Early methods of measuring airborne concentrations were based on the collection process of impingement in water. The collected particles in liquid suspension were then counted microscopically. With this method, all visible particles, fibrous and nonfibrous, were counted. This method was nonspecific as to the makeup of the particulate sampled and did not distinguish between the fibrous and nonfibrous component of the collected dust. This impinger method became the standard dust sampling instrument in the 1930's and was the first basis for the first standards for asbestos dust in the United States.
Through the years, many methods of measuring airborne particulate such as asbestos have been used. Based on the data developed, the National Institute for Occupational Safety and Health (NIOSH) concluded that measurement of the fibrous component of the dust was more appropriate for characterizing workplace atmospheres relative to the health hazard from asbestos exposure. NIOSH further concluded that the best index of asbestos exposure was the concentration of fibers longer than 5 .mu.m collected on membrane filters, such fibers being counted using phase contrast microscopy.
The first version of the membrane filter procedure for monitoring airborne asbestos was published by NIOSH in 1968. After years of study and refinement, NIOSH published a formal method in 1976. This standardized method was developed and adopted to better control the sources of variability in the procedure, and improve the accuracy and the precision of the data collected. The procedure specified particulate collection on a 37 mm diameter mixed cellulose ester filter mounted in an open face plastic filter holder.
As the levels of acceptable exposure to asbestos were lowered, investigation continued concerning ways to reduce the variability of the results and improve the capability for measuring lower levels of airborne asbestos. A modification of the sampling device that proved effective involved use of a smaller filter, 25 mm in diameter. This, in effect, concentrated the sample in a smaller area thereby increasing the sensitivity of the analytical method.
In 1984, NIOSH issued a new sampling and analytical procedure (Method 7400) that utilized the 25 mm cassette. Later a protective 50 mm extension cowl was added to the open-faced holder. However, electrostatic effects introduced by this cowl caused a more non-uniform deposition of fibers on the filter. Since only a fraction of the filter is actually analyzed, any variability in this distribution of fibers on the filter vastly increases the variability of the estimation of the total number of fibers on the entire filter, and therefore causes an underestimation of airborne asbestos fiber concentration. To minimize this, an electrically conductive cowl was introduced in an attempt to minimize the electrostatic effect and produce a more uniform fiber distribution.
Through the years, government regulations have continuously lowered the level of acceptable exposure to airborne asbestos fiber. Thus, better control of the accuracy and variability of the monitoring procedure has become more and more important. Since the variability of the procedure increases drastically as the number of fibers counted decreases, reliable determination of these lower levels has been difficult and problematic.
Fiber distribution on the filter is a main factor contributing to the overall procedure variability. A key to improving the procedure is reducing non-uniform deposition of the fibers on the filter. Even with the electrically conductive cowl, non-uniform fiber deposition has remained a major source of variability in the procedure.
Currently, NIOSH, the Occupation Safety and Health Administration (OSHA), and the Environmental Protection Agency (EPA) have published procedures for sampling and analysis of airborne asbestos fibers. The sampling technique for these procedures generally involves drawing an accurately known volume of air through a membrane filter which is held in a cassette. A portion of the filter is then analyzed for determination of the number of fibers retained on the surface of the filter.
One method of analysis of the filter utilizes phase contrast microscopy to count all fibers meeting specified dimensional characteristics. Other methods have also been developed utilizing the more sophisticated technique of electron microscopy. While the electron microscope has the ability to differentiate asbestos particles from non-asbestos particles, it utilizes a smaller area of the filter for examination as compared to the phase contrast microscopy. Therefore, the actual part of the filter analyzed is even less representative of the total filter and minimizing non-uniform fiber deposition on the filter becomes even more important.